Directional power detection by quadrature sampling

ABSTRACT

Power measurement and control in transmission systems are affected by changes in load conditions. A method and system are provided for detecting and controlling power levels independent of such load conditions.

REFERENCE TO EARLIER FILED APPLICATION

This application claims the benefit of and incorporates by referenceU.S. Provisional Application Ser. No. 60/707,671, filed Aug. 12, 2005,and entitled “DIRECTIONAL POWER DETECTION BY QUADRATURE SAMPLING”.

INTRODUCTION Field of the Invention

The present invention relates to signal transmissions and, inparticular, to measurement and control of signal transmission power.

BACKGROUND

In order to have power measurement and control, transmitters areconfigured with power control feedback loops responsive to powerdetectors. In common configurations for high (e.g., microwave) frequencybands, the power level is measured by a detector in a waveguide which isconnected between the output of the power amplifier and the load.

In general, waveguides are used for transporting high frequency signals,in part because of their low-loss characteristics and ability to handlehigh power. Waveguide components are configured in a number ofgeometries, examples of which include ‘parallel’ with a pair of plates,‘co-planar’ with a thin slot in the ground plane of one side of adielectric substrate with or without a conductor in the slot,‘dielectric’ with a dielectric ridge on a conductor substrate, ‘ridge’with conducting ridges on the top and/or bottom walls, and ‘rectangular’with a parallel-piped structure of a substantially rectangular crosssection. Thus, although the discussion here examines rectangularwaveguides, other waveguide may be suitable for power measurement.

One approach to power measurement can be described as the single probeapproach, as shown in FIG. 1. The waveguide component is defined by itstop, bottom, input and load planes, 12 a–d, respectively. The waveguidehas a single slot 14 in the bottom plane and a single probe 15 formeasuring the power level protrudes into the waveguide through this slot14. The probe 15 is often made of a conductive material and thepotential generated thereby drives the detector diode 16. The output ofthe detector diode 16 is connected to a buffer amplifier 18 in order toisolate the detector diode from downstream components (not shown) andprevent their interference with its signal integrity.

As shown, forward signals traverse the waveguide from the input plane 12c to the load plane 12 d. Ideally, there would be a perfect impedancematch between the waveguide and the load (antenna or test equipment notshown) and the entire signal energy would be transferred from thewaveguide to the load. In reality, however, the match is imperfect andresults in reflections of the forward signals from the load plane 12 d.The opposite-traveling reflected signals interfere with the forwardsignals and this produces a new wave pattern known as standing waves,which is what the probe 15 ultimately measures.

The amplitude of the standing waves is affected by the degree ofinterference of the reflected signals with the forward signals which isbased on the degree of mismatch between the waveguide and the load.Then, because with the single probe configuration there is no isolationfrom the load mismatch, this measurement is strongly influenced byvariations in the load conditions.

A second approach, described as a directional waveguide coupler,attempts to solve the problems associated with the unreliable powermeasurement inherent in the single probe configuration. FIG. 2illustrates the directional waveguide coupler.

The directional waveguide 21 is designed for a particular frequency bandwith top, bottom, input and load planes 22 a–d, respectively, and withthe slots 24 a and 24 b in the bottom plane 22 b spaced apart a quarterwavelength (or 90°). Attached to the bottom plane of the waveguide andfacing the slots 24 a and 24 b is a coupler 23, also configured as awaveguide. The coupler 23 has a waveguide termination plate 26 and abottom plate 25 with a slot 28 through which the power probe 29protrudes. As before, the power probe 29 is connected to a detectordiode 32 which is, in turn, connected to the buffer amplifier 34 toproduce the detector output while isolating it from downstream stages.

Under ideal load conditions there would be a perfect match between thewaveguide and the load (antenna or test equipment not shown), and theload plane 22 d would transfer the forward signals from the waveguide tothe load without losses. In reality, the load conditions are not perfectbecause of the load-waveguide impedance mismatch and the load plane 22 dreflects the forward signals. The reflected waves interfere with theforward waves and whenever two waves of similar frequency travel in amedium in opposite directions standing waves are formed. Thus, the loadplane acts as a constructive or destructive reflector based on itsposition relative to the resultant standing waves cycle. The sameapplies to the signals passing to the coupler through the slots 24 a and24 b.

The forward signals that pass through slots 24 a and 24 b, respectively,converge at the probe 29 in phase. This is because the forward signalsmoving through the waveguide 21 and slot 24 b and those moving throughslot 24 a and the coupler 23 travel the same respective quarterwavelength (90°) distance. At the same time, reflected signals whichpass through slot 24 a travel the quarter wavelength (90°) distancetwice, once in the direction toward slot 24 a and once in the oppositedirection toward the probe 29. In other words, reflected signals thatpass through slot 24 a are 180° out of phase relative to the reflectedsignals that pass through slot 24 b.

It is noted that a full cycle of the wave is comparable to a full circleof 360°, and any fraction of the circle in degrees is comparable to afraction of the wave cycle which is the phase. When the forward andreflected signals are in phase (0° or 360° phase difference), theinterference is constructive and produces a standing wave which is thesum of both (with twice the amplitude); and the interference isdestructive when they are out of phase from each other. The phase shift(P) between the opposite-traveling waves can be 0<P<360°, where a 180°phase shift results in mutual cancellation of these waves.

Thus, the reflected signals converge at the probe 29 at 180° out ofphase and cancel each other. Ideally, the probe 29 reads the magnifiedforward signals and none of the reflected signals. In reality, however,there is an imperfect match at the waveguide termination plate 26 andsome of the reflected signals do end up converging at the probe withless or more than 180° phase shift.

The reason for this imperfection is any inaccuracy in the complexmechanical structure of the waveguide and coupler. Indeed, any variationin the operating frequency and/or the mechanical dimensions or materialof the waveguide and coupler components can create a mismatch and, as aresult, introduce some of the reflected waves at the probe 29. Inparticular, the frequency dependent waveguide termination plate designcalls for different types of material to achieve the desiredperformance. Moreover, manufacture of the waveguide and coupler involvesnon-flexible frequency-dependent mechanical and electrical design forachieving performance such as isolation and power coupling. The two-partdirectional waveguide structure is hard to build and is even harder toreplicate in commercial quantities.

SUMMARY

In view of the foregoing, the present invention proposes solutions thataddress this and related issues. These solutions include systems,devices and methods that are provided in accordance with the principlesand various embodiments of the present invention.

As shown and broadly described herein, one embodiment is a device forpower detection. This device includes a waveguide, a pair of probes, aquarter wavelength delay component, a power combiner, a detector diodeand a carrier on which the pair of probes, quarter wavelength delaycomponent, power combiner and detector diode are laid out. The waveguideis configured for transporting forward and reflected waves of a signalwith a corresponding wavelength and a power level. For the purpose ofpower detection, the waveguide has a pair of slots spaced apart by aquarter of the wavelength. The pair of probes protrudes into thewaveguide through the pair of slots for probing the signal and measuringits power level. The pair of probes, quarter wavelength delay component,power combiner and detector diode are laid out on the carrier in aconfiguration where the forward waves converge at the power combinerconstructively and the reflected waves converge at the power combinerdestructively. This way, the measured power level is substantiallyindependent from load condition variations.

It is noted that the carrier is a printed circuit board or a substrate.The substrate can be integrated into an integrated circuit or any othersuitable configuration for high frequency (particularly microwave)applications. In the case where the carrier is a substrate it may or maynot be partially encapsulated but the circuit components that need to beexposed, such as the probes, are properly exposed to the waves.

Typically, the waveguide has top, bottom, input and load planes, whereinthe pair of slots is located in the bottom plane such that one of theslots and, in turn, one of the probes, are closer to the input plane. Inthis case, the configuration is one in which the detector diode isconnected to the power combiner for receiving a signal proportionate tothe measured power level and in which the power combiner is connected tothe pair of probes. One side of the power combiner is connected via thequarter wavelength delay component to the probe which is closer to theinput plane.

Another device for power detection is configured with a pair of membersand a detector circuit carrier. In particular, the pair of members isdetachably joined to form a body with a duct, wherein the duct defines awaveguide for transporting forward and reflected waves of a signal witha corresponding wavelength and a power level. As before, the waveguidehas a pair of slots spaced apart by a quarter of the wavelength. Thedetector circuit carrier is inserted between the joined pair of membersand fits removably therebetween. The detector circuit carrier has a pairof probes, a quarter wavelength delay component, a power combiner, and adetector diode, wherein the pair of probes protrudes into the waveguidethrough the pair of slots for probing the signal and measuring its powerlevel. Moreover, the pair of probes, quarter wavelength delay component,power combiner and detector diode are laid out on the detector circuitcarrier in a configuration where the forward waves converge at the powercombiner constructively and the reflected waves converge at the powercombiner destructively, whereby the measured power level issubstantially independent from load condition variations. Again, thedetector circuit carrier is a printed circuit board or a substrate asexplained above.

In yet another embodiment, a power detection and control loop in atransmission system includes a waveguide between an output stage and aload, a detector circuit for measuring the power level substantially atthe load and a control circuit. The output stage produces a signal witha corresponding wavelength and power level, and the waveguide transportsforward and reflected waves of the signal. Again, the waveguide has apair of slots spaced apart by a quarter of the wavelength. The detectorcircuit is laid out on a circuit carrier and has a pair of probes, aquarter wavelength delay component, a power combiner, and a detectordiode, wherein the pair of probes protrudes into the waveguide throughthe pair of slots for probing the signal and measuring its power level.The pair of probes, quarter wavelength delay component, power combinerand detector diode are laid out on the detector circuit carrier in aconfiguration where the forward waves converge at the power combinerconstructively and the reflected waves converge at the power combinerdestructively, whereby the power measurement is substantiallyindependent from load condition variations. The control circuit has abuffer amplifier linked to stages responsive to the measured power leveland operatively linked to the output stage for controlling its gain and,in turn, the power level of the signal. Incidentally, the load is anantenna or a dummy load in a test device and, as before, the circuitcarrier is a printed circuit board or a substrate.

In accordance with yet another embodiment of the invention, a method fordetecting power in a transmission system includes a number of steps. Onestep involves inserting a waveguide between an output stage and a loadfor transporting forward and reflected waves of a signal with acorresponding wavelength and a power level. For this method, as with thedevices above, the waveguide has a pair of slots spaced apart by aquarter of the wavelength. Another step involves inserting, through thepair of slots, a pair of probes into the waveguide, the pair of probesbeing capable of probing the signal and measuring the power level.Another step is where a quarter wavelength delay component is introducedinto a measured signal path between a first one of the pair of probesand one side of a power combiner, the other side of the power combinerbeing directly connected to the a second one of the pair of probes.Then, another step involves detecting a signal proportionate to themeasured power level, the signal being detected by a diode whichreceives the signal from the power combiner. Once again, the pair ofprobes, the quarter wavelength delay component, the power combiner andthe diode are laid out on a circuit carrier in a configuration where theforward waves converge at the power combiner constructively and thereflected waves converge at the power combiner destructively, wherebythe measured power level is substantially independent from loadcondition variations.

In accordance with yet another embodiment, a method for detecting andcontrolling power in a transmission system includes the step ofinserting a waveguide between an output stage and a load fortransporting forward and reflected waves of a signal with acorresponding wavelength and a power level. As with the precedingembodiments, the waveguide has a pair of slots spaced apart by a quarterof the wavelength. This method further includes the step of inserting,through the pair of slots, a pair of probes into the waveguide. This isso that the pair of probes can probe the signal and measure the powerlevel. This method additionally includes the step of introducing aquarter wavelength delay component into a measured signal path between afirst one of the pair of probes and one side of a power combiner, theother side of the power combiner being directly connected to the asecond one of the pair of probes. With this approach, a signalproportionate to the measured power level is detected. The signal isdetected by a diode which receives the signal from the power combiner.In this instance, as in the others, the pair of probes, the quarterwavelength delay component, the power combiner and the diode are laidout on a circuit carrier, a printed circuit board or a substrate, in aconfiguration where the forward waves converge at the power combinerconstructively and the reflected waves converge at the power combinerdestructively. This way, the measured power level is substantiallyindependent from load condition variations. The detected signal is fedto downstream stages that are operatively linked with the output stageto control its gain and, in turn, the power level in response to themeasured power level.

One benefit derived from the present invention as broadly describedherein is simplicity of implementation. Another benefit derived from thepresent invention is lower cost to manufacture for commercialapplications. And, even with this simpler configuration, the presentinvention advantageously provides a more reliable power measurement andcontrol. In sum, these and other features, aspects and advantages of thepresent invention will become better understood from the descriptionherein, appended claims, and accompanying drawings as hereafterdescribed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings which are incorporated in and constitute apart of this specification illustrate various aspects of the inventionand together with the description, serve to explain its principles.Wherever convenient, the same reference numbers will be used throughoutthe drawings to refer to the same or like elements. The drawingsinclude:

FIG. 1 illustrates a conventional single probe configuration;

FIG. 2 illustrates a conventional directional waveguide couplerconfiguration;

FIG. 3 illustrates a directional waveguide power detector configured inaccordance with an embodiment of the present invention; and

FIG. 4 is a perspective view of the directional waveguide of FIG. 3.

DETAILED DESCRIPTION

The present invention is based, in part, on the observation thattransmitters of radio frequency signals are calibrated for particularload conditions. In a typical situation, a transmitter is calibratedwith test equipment as the load and then used with an antenna as theload. The load conditions created by the test equipment are notnecessarily exactly the same as the load conditions created by theantenna, and, moreover, different antennas have slightly differentcharacteristics and may create different load conditions at the outputof the transmitter. In some situations, the manufacturer may calibratethe transmitter with one kind of test equipment and a complianceverification laboratory may test the transmitter with another kind oftest equipment (e.g., for FCC rules compliance). Transmitter loadconditions may vary also with environmental changes such as temperaturesand humidity variations. For this reason the present invention looked atways to substantially overcome variations in load conditions and therebyimprove power measurement and control in transmission systems. We willexamine such ways with the examples that follow.

In general, because it recognizes that load conditions are imperfect andoften result in standing waves produced from reflected signalsinterfering with forward signal, the present invention proposes tosubstantially cancel the effects of the reflected waves. Specifically,the present invention proposes to converge reflected waves which are outof phase at substantially 180° and thus cancel each other.

One approach for implementing this involves quadrature sampling in adirectional waveguide. FIG. 3 illustrates a directional waveguide forpower detection by quadrature sampling.

As shown, the power detection system 100 includes a directionalwaveguide defined by top, down, input and output planes, 102 a–d,respectively. The bottom plane 102 b has two slots 104 a and 104 bspaced apart a quarter wave distance (90°), based on the frequency band.Two probes 106 a and 106 b (labeled P1 and P2, respectively) protrudethrough the slots into the waveguide. The probes are therefore alsospaced apart a quarter wave distance, or 90°. The physical dimensions ofthe waveguide and, in particular, the distance between the slots 104 aand 104 b depend on the frequency range of transmission. Thus, forinstance, if the transmission frequency is 50 GHz and the bandwidth is10% of the transmission frequency, i.e., +/−2.5 GHz, a quarterwavelength would be 1.5 mm.

In this configuration, the probes, P1 and P2, are passive devices suchas conductors (traces) on a printed circuit board (PCB) 120. The PCB isshaped to allow passage of the two probes through the slots 104 a and104 b. Then, in addition to the probes, the PCB 120 holds detectorcircuit components such as a 90° delay line 108, a power combiner 110and a detector diode 112. The power detector circuit on the PCB isformed with the probe P1 connected to one side of the power combiner viathe 90° delay line and with the probe P2 connected to the other side ofthe power combiner. The detector diode 112 is connected across the powercombiner 110 to receive a signal which represents the measured power.The power combiner in this circuit is a passive circuit such as aresistive connection that produces a voltage drop proportionate to thecurrent induced from the power measured by the probes P1 and P2.

The buffer amplifier 114 and downstream stages (not shown) are locatedoff the PCB 120. The buffer amplifier protects the detector diode fromthe effects of downstream stages in order to maintain the diode's signalintegrity and reliably correlate the output of the diode with themeasured power.

In operation, the forward signals are any type of transmitted signals ata particular frequency range, having a particular power level and beingmodulated if they carry any information. Un-modulated signals with aparticular frequency do not contain any information and they aretypically known as the carrier waves. Modulated signals carryinformation and they are created by various modulation techniquesexamples of which include AM (amplitude modulation), FM (frequencymodulation), QAM (quadrature amplitude modulation), and PWM (pulse widthmodulation). The forward signals travel from the input plane 102 ctoward the output plane 102 d and because of imperfect load conditionsreflected signals travel in the opposite direction. Both forward andreflected signals are intercepted by the probes P1 and P2, which arelocated 90° apart, and converge at the power combiner.

As they travel through the waveguide, forward waves intercepted by probeP1 pass through the 90° delay line and thus incur a 90° delay. At thesame time, forward signals intercepted by probe P2 pass directly to thepower combiner, but they incur a 90° delay in reaching probe P2 becauseof the 90° distance between probe P1 and P2. In other words, becausethey are equally delayed by 90°, the forward signals intercepted byprobes P1 and P2 converge at the power combiner in phase relative toeach other. This means that the forward signals' convergence isconstructive and the resulting signal is the sum of both.

By comparison, the reflected signals converge at the power combiner atopposite phases (180°) relative to each and their convergence isdestructive. More specifically, reflected waves intercepted by probe P2pass directly to the power combiner while reflected waves intercepted byprobe P1 travel 180° before they reach the power combiner (90° distanceto P1 and 90° delay at the delay line). Signals converging at 180° phasedifference cancel each other. Therefore, the destructive convergence ofthe reflected signals results in them canceling each other and notaffecting the power measurement. In other words, the measured power aspresented by the voltage across the power combiner is substantially freefrom load condition variations. The measured power is then reliablydetected by the detector diode 112 and the value is passed along via thebuffer amplifier 114 to downstream stages (of the power control loop).

It is noted that the frequency range is scalable to other, higherfrequencies simply with changes to the PCB layout design and changes tothe waveguide dimensions and distance between the slots. In essence,there would be one set of dimensions for each frequency, but thefundamental design is similar for the various frequencies. The ease withwhich a PCB can be designed and made is one advantage of the presentinvention.

It is further noted that the depth of insertion of the probes into thewaveguide controls the sensitivity of the detector circuit (i.e., thepower level detection voltage at the power combiner). Hence, the easewith which the PCB can be adjusted to achieve the proper depth ofprotrusion into the waveguide is yet another advantage of the presentinvention. Furthermore, the PCB can be made sufficiently small that itfits easily inside the waveguide body.

FIG. 4 is an isometric view of a waveguide, taken apart, and a PCB withthe power detection circuit. In this illustration, the waveguide isproduced when the two semi-circular members 101 a and 101 b are joined.The material these members are made out of is suitable for microwaveapplications and is therefore suitable for producing the waveguide. Whenjoined, the two members form a cylinder with a duct which, in this case,has a rectangular cross section and is substantially aligned with theaxis of the cylinder. The length of the cylinder determines the lengthof the duct and, in turn, the length of the waveguide (as necessary forthe particular frequency band). The shape and dimensions of the ductdefine the walls of the waveguide and particularly the top, bottom,input and load planes 102 a–d. The bottom plane 102 b has two notchesthat define the slots 104 a and 104 b through which the probes 106 a and106 b can protrude into the waveguide. Being smaller than the length ofthe waveguide, the distance between the slots, and in turn the probes,is set to a quarter wavelength (90°) which varies with the transmissionfrequency band. One or both members accommodate the PCB and the slots.Specifically, one or both members 101 and 101 b have a detector notchextending below the bottom plane (not shown) for fitting the PCB withthe power detection circuitry between them when the members are joinedsuch that the probes are allowed to protrude through the slotssufficiently to produce the desired sensitivity. Moreover, the notchesthat define the slots 104 a and 104 b in the bottom plane 102 b arecarved out of one or both members, depending on whether the detectornotch is provided in one or both members.

As mentioned before, the detector circuitry is mounted on the PCB andbecause the circuit components are small the PCB dimensions can be smallas well. What changes with frequency is the waveguide dimensions and thedistance between the probes and the slots. The frequency change requiresvery simple redesign of the PCB layout and mechanical dimensions of themembers that produce the waveguide. Therefore, this configuration iseasy to manufacture in commercial applications and the results areeasily repeatable.

In sum, the present invention provides ways in which reliable powerdetection and control can be achieved despite variations in loadconditions; and the mechanical-electrical configuration of the powerdetection system is relatively simple and less costly to produce. Thus,although the present invention has been described in considerable detailwith reference to certain preferred versions thereof, other versions arepossible. In other words, the spirit and scope of the appended claimsshould not be limited to the description of the preferred versionscontained herein.

1. A device for power detection, comprising: a waveguide fortransporting forward and reflected waves of a signal with acorresponding wavelength and a power level, the waveguide having a pairof slots spaced apart by a quarter of the wavelength; a pair of probesprotruding into the waveguide through the pair of slots for probing thesignal and measuring its power level; a quarter wavelength delaycomponent; a power combiner; a detector diode; and a carrier on whichthe pair of probes, quarter wavelength delay component, power combinerand detector diode are laid out in a configuration where the forwardwaves converge at the power combiner constructively and the reflectedwaves converge at the power combiner destructively, whereby the measuredpower level is substantially independent from load condition variations.2. The device of claim 1, wherein the carrier is a printed circuit boardor a substrate.
 3. The device of claim 1, wherein the configuration isone in which the detector diode is connected to the power combiner forreceiving a signal proportionate to the measured power level and inwhich the power combiner is connected to the pair of probes, one side ofthe power combiner being connected to one probe of the pair of probesvia the quarter wavelength delay component.
 4. The device of claim 1,wherein the waveguide has top, bottom, input and load planes, whereinthe pair of slots is located in the bottom plane such that one of theslots and, in turn, one of the probes, are closer to the input plane. 5.The device of claim 4, wherein the configuration is one in which thedetector diode is connected to the power combiner for receiving a signalproportionate to the measured power level and in which the powercombiner is connected to the pair of probes, one side of the powercombiner being connected via the quarter wavelength delay component tothe probe which is closer to the input plane.
 6. The device of claim 4,wherein the waveguide has a rectangular cross section.
 7. The device ofclaim 1, wherein the pair of probes is conductive traces or strips. 8.The device of claim 1, wherein the power combiner is a resistiveconnection.
 9. A device for power detection, comprising: a pair ofmembers detachably joined to form a body with a duct, wherein the ductdefines a waveguide for transporting forward and reflected waves of asignal with a corresponding wavelength and a power level, the waveguidehaving a pair of slots spaced apart by a quarter of the wavelength; anda detector circuit carrier inserted between the joined pair of membersand fitting removably therebetween, the detector circuit carrier havinga pair of probes, a quarter wavelength delay component, a powercombiner, and a detector diode, wherein the pair of probes protrudesinto the waveguide through the pair of slots for probing the signal andmeasuring its power level, and wherein the pair of probes, quarterwavelength delay component, power combiner and detector diode are laidout on the detector circuit carrier in a configuration where the forwardwaves converge at the power combiner constructively and the reflectedwaves converge at the power combiner destructively, whereby the measuredpower level is substantially independent from load condition variations.10. The device of claim 9, wherein the detector circuit carrier is aprinted circuit board or a substrate.
 11. The device of claim 9, whereinthe configuration is one in which the detector diode is connected to thepower combiner for receiving a signal proportionate to the measuredpower level and in which the power combiner is connected to the pair ofprobes, one side of the power combiner being connected to one probe ofthe pair of probes via the quarter wavelength delay component.
 12. Thedevice of claim 9, wherein the waveguide has top, bottom, input and loadplanes, wherein the pair of slots is located in the bottom plane suchthat one of the slots and, in turn, one of the probes, are closer to theinput plane.
 13. The device of claim 12, wherein the configuration isone in which the detector diode is connected to the power combiner forreceiving a signal proportionate to the measured power level and inwhich the power combiner is connected to the pair of probes, one side ofthe power combiner being connected via the quarter wavelength delaycomponent to the probe which is closer to the input plane.
 14. Thedevice of claim 12, wherein the waveguide has a rectangular crosssection.
 15. The device of claim 9, wherein the body is a cylinder andthe duct is substantially aligned with the axis of the cylinder.
 16. Thedevice of claim 9, wherein the pair of probes is conductive traces orstrips.
 17. The device of claim 9, wherein the power combiner is aresistive connection.
 18. A power detection and control loop in atransmission system, comprising: a waveguide inserted between an outputstage and a load of a transmission system, the output stage producing asignal with a corresponding wavelength and a power level and thewaveguide transporting forward and reflected waves of the signal, thewaveguide having a pair of slots spaced apart by a quarter of thewavelength; a detector circuit for measuring the power level of thesignal, the detector circuit being laid out on a circuit carrier andhaving a pair of probes, a quarter wavelength delay component, a powercombiner, and a detector diode, wherein the pair of probes protrudesinto the waveguide through the pair of slots for probing the signal andmeasuring its power level, and wherein the pair of probes, quarterwavelength delay component, power combiner and detector diode are laidout on a detector circuit carrier in a configuration where the forwardwaves converge at the power combiner constructively and the reflectedwaves converge at the power combiner destructively, whereby the measuredpower level is substantially independent from load condition variations;and a control circuit having a buffer amplifier linked to stagesresponsive to the measured power level and operatively linked to theoutput stage for controlling its gain and, in turn, the power level ofthe signal.
 19. A power detection and control loop in a transmissionsystem as in claim 18, wherein the buffer amplifier provides a bufferbetween the detector diode and the stages.
 20. A power detection andcontrol loop in a transmission system as in claim 18, wherein the loadis an antenna or a dummy load in a test device.
 21. A power detectionand control loop in a transmission system as in claim 18, wherein thecircuit carrier is a printed circuit board or a substrate.
 22. A powerdetection and control loop in a transmission system as in claim 18,wherein the configuration is one in which the detector diode isconnected to the power combiner for receiving a signal proportionate tothe measured power level and in which the power combiner is connected tothe pair of probes, one side of the power combiner being connected toone probe of the pair of probes via the quarter wavelength delaycomponent.
 23. A power detection and control loop in a transmissionsystem as in claim 18, wherein the waveguide has top, bottom, input andload planes, wherein the pair of slots is located in the bottom planesuch that one of the slots and, in turn, one of the probes, are closerto the input plane which is facing the output stage.
 24. A powerdetection and control loop in a transmission system as in claim 23,wherein the configuration is one in which the detector diode isconnected to the power combiner for receiving a signal proportionate tothe measured power level and in which the power combiner is connected tothe pair of probes, one side of the power combiner being connected viathe quarter wavelength delay component to the probe which is closer tothe input plane.
 25. A power detection and control loop in atransmission system as in claim 23, wherein the waveguide has arectangular cross section.
 26. A power detection and control loop in atransmission system as in claim 18, wherein the pair of probes isconductive traces or strips.
 27. A power detection and control loop in atransmission system as in claim 18, wherein the power combiner is aresistive connection.